1. Field of the Invention
The present invention relates to a blood-pressure monitoring apparatus which monitors a blood pressure of a living subject based on information relating to propagation of a pulse wave through an arterial vessel of the subject.
2. Related Art Statement
As information relating to a pulse wave which propagates through an arterial vessel of a living subject, there are known a pulse-wave-propagation time DT and a pulse-wave-propagation velocity V.sub.M (m/s). The pulse-wave-propagation time DT is a time which is needed by a pulse wave to propagate between two different positions of the arterial vessel. Additionally, it is known that the above pulse-wave-propagation-relating information is, within a predetermined range, substantially proportional to the blood pressure ("BP", mmHg) of the living subject. Therefore, there has been proposed a BP monitoring apparatus which determines, in advance, coefficients .alpha., .beta. in the following expression: EBP=.alpha.(DT)+.beta. (where .alpha. is a negative constant and .beta. is a positive constant), or EBP=.alpha.(V.sub.M)+.beta. (where .alpha. and .beta. are positive constants), based on two measured BP values of the subject and two measured pulse-wave-propagation time values (DT) or two measured pulse-wave-propagation velocity values (V.sub.M), iteratively determines an estimated BP value EBP of the subject, based on each piece of subsequently obtained pulse-wave-propagation-relating information, according to the above-indicated first or second expression, and starts a BP measurement using an inflatable cuff when an estimated BP value EBP is judged as being abnormal.
Meanwhile, at least two measured BP values and at least two pieces of obtained pulse-wave-propagation-relating information are needed for determining a relationship between pulse-wave-propagation-relating information and estimated BP value EBP represented by the first or second expression, because each of the first and second expressions includes two unknown constants. In order to determine an accurate relationship, it is desirable that the two measured BP values have the greatest possible difference. However, the conventional BP monitoring device carries out the second BP measuring operation using the inflatable cuff, irrespective of what BP value the subject currently has. Accordingly, the accuracy of the determined relationship may be insufficient. If many BP measuring operations each using the cuff are repeated till two BP values having a sufficiently great difference are obtained, it takes a long time and results in giving an unnecessary load to the subject.